The magnetic susceptibilities of seven different nontronites in their natural oxidation states were measured between 5 and 100 K. Results revealed that the magnetic exchange interaction in all samples was antiferromagnetic, except no clear minimum occurred at the Néel temperature. Possible explanations for this phenomenon which are discussed include magnetic dilution due to isomorphous substitution, and antiferromagnetic frustration due to either non-centrosymmetric distribution of octahedral Fe3+ or tetrahedral Fe3+ substitution. A computer simulation model was developed to demonstrate the effects of these variables on long-range magnetic ordering. Magnetic dilution and tetrahedral Fe3+ content could explain the anomalous antiferromagnetic behaviour in some, but not all, samples. The non-centrosymmetric model is the only one which explains the behaviour of all samples. In this model, at least 13% of the octahedral Fe3+ would occupy trans-dihydroxide sites, with the balance in cis sites. Magnetic frustration occurs because two Fe3+ neighbours of a third Fe3+ ion are also neighbours to each other, making the simultaneous satisfaction of all antiferromagnetic exchange interactions impossible.

A pillared smectite clay has been formed by treating Na-montmorillonite with a hydroxy-aluminium solution with an OH/Al ratio of two. Heating the pillared clay for four h at 500°C reduces the interlayer spacing, measured at ambient humidity, from ∼ 9·4 to 8·0 Å. Hydrated VO2+ (and Mg2+) ions have been introduced into the region between the pillars by immersing the pillared clay in VOSO4-MgCl2 solutions and EPR measurements show that in general both immobile and mobile VO2+ ions are present. The immobile ion is probably VO2+(OH)x(H2O)5·x where x = 2–4, and is adsorbed on the pillars with a concentration that saturates at about 1 per 8 Al13 pillars in the unfired clay, and 1 per 4·3 pillars in the fired clay. The mobile species is VO2+(H2O)5 and for both fired and unfired clay in the wet state at room temperature it has a rotational correlation time of (9 ± 1) × 10−11s rad−1.

Isocitric acid oxidative decarboxylation was realized in the absence and in the presence of homoionic Na+-, Mn2+-, and Cu2+-montmorillonite. The catalytic activity of the clay depends upon the nature of the interlayer exchangeable cation. Isocitric acid is transformed into α-ketoglutaric acid under the action of the clay mineral saturated with Na+ cations which do not form a complex with the isocitrate anion. Nevertheless, the reaction rate is very much lower than in the presence of the enzymatic system (isocitrate dehydrogenase enzyme and nicotinamide adenine dinucleotide phosphate coenzyme). The reaction mechanism in the presence of clay is given showing the different steps of the transformation.

An empirical method to estimate rapidly both mean total layer charge and location of charge in smectites and vermiculites is presented, involving exchange with a long-chain alkylammonium ion before and after the Greene-Kelly test. The method utilizes an empirically determined relationship between XRD basal spacings and the mean layer charges calculated from the conventional n-alkylammonium ion-exchange technique using simple linear regression models to describe the relationships for both monolayer-bilayer and bilayer-pseudotrimolecular layer transitions. The method provides a rapid estimation of these charge parameters and facilitates the proper identification of both smectites and vermiculites consistent with the AIPEA criteria for classification based on total layer charge. It is considered superior to conventional methods of differentiation based mainly on expansion and collapse behaviour.

Changes which occur during the natural weathering of biotite in granite gneiss and associated soils are measured by microanalysis and illustrated by SEM. Biotite weathers through a series of interstratified minerals to vermiculite and/or smectite phases which decompose rapidly to kaolinite. Both vermiculite and smectite phases appear to be dioctahedral, on the basis of chemical compositions derived from microprobe data. Weathering products are first apparent on the edges of laminae, where interstratified minerals are formed at right angles to both the edge face and the cleavage. Weathering soon develops along cleavage planes, initially most strongly near the edges of flakes, but then permeating extensively into the body of flakes and subdivided segments. The orientation of interstratified minerals and kaolinite within cleavages is parallel to the cleavage.

To investigate the influence of carbonate on the formation of goethite and lepidocrocite, ∼200 samples were synthesized by oxidizing FeCl2 solutions with air/CO2 gas mixtures at ambient temperature and pH 6 and 7. The proportion of lepidocrocite in the lepidocrocite/goethite mixtures (Lp/(Lp + Gt)) decreased from 100 to 0% with increasing in solution and with decreasing average oxidation rate (AOR). These two parameters explained 81% of the variation of Lp/(Lp + Gt)). At a given , more goethite was formed at pH 6 than at pH 7. The Lp + Gt mixtures contained 0–8 mg g−1 carbon (Ct) which could not be removed by washing. Ct reached apparent saturation at a equilibrium concentration of ∼6–8 and 60–80 mmol l−1 at pH 6 and pH 7, respectively. In a plot of Ct vs. Lp/(Lp + Gt) all data fell on the same line irrespective of oxidation parameters (pH, AOR). IR spectra showed two broad bands at ∼1300 and 1500 cm−1 which can be assigned to distorted carbonate adsorbed at the goethite surface. Identical bands were also found in a young, poorly crystalline goethite formed from coal mine drainage in Ohio. It is suggested that carbonate anions direct the polymerization of the double bands of FeO3(OH)3 octahedra common to both minerals toward a corner sharing arrangement, and thereby to goethite, whereas chloride permits edge-sharing as in lepidocrocite.

A unified system of vector representation of chemical composition is proposed for the phyllosilicates based on projection of the composition, as given by crystallochemical formula, onto a field with orthogonal axes chosen for octahedral divalent cations, R2+, and Si (X, Y, respectively), and oblique axes for octahedral trivalent cations, R3+, and vacancies, □, (V, Z, respectively). Point coordinates for each set of axes were used to define the direction and length of the unit vectors for phyllosilicates belonging to different groups. Parallel to these fundamental directions the composition isolines were drawn in the projection fields. Applied to micas, this system enables control of the chemical composition by the general crystallochemical formula covering all varieties of Li-free dioctahedral and trioctahedral micas:

where z (number of vacancies) = (y-x+ m)/2; m (layer charge) =1; u+y+z = 3. There is a similar formula for vacancy-free lithian micas:

where w = m — x+y;m=1; u+y+w = 3, and for Li-free brittle micas:

where z = (y — x+m)/2; m = 2; u+y+z = 3. Projection fields were used to classify micas.

The classification and nomenclature of chlorites have been critically reviewed. A new classification based on the unified projection system of chemical composition has been proposed in which for the first time all trioctahedral, di-trioctahedral and dioctahedral species are included. The chemical composition of chlorites is controlled by the general crystallo-chemical formula: This is superior to the known formulae for the trioctahedral and dioctahedral chlorites. It has been shown that the chlorite end-member compositions of Bayliss (1975) project in one point of the projection field leaving many end-member compositions unnamed. As they correspond to the names used previously, a proposal is made to restore these names.

A unified system of projection of chemical composition, prepared initially for micas and chlorites, has been applied to minerals of the serpentine-kaolinite group. It has been shown that the chemical composition in the projection field is controlled by the formula, the unit of which is: (Si(2-x)Alx)O5(OH)4, where u+y+z=3, z=y-x. Using projection fields for different chemical systems it has been shown that among the most important end-members are kaolinite minerals, true serpentines, berthierine, brindleyite, amesite, cronstedtite, greenalite, nepouite and their analogues having different substitutions in the octahedral sheets.

Micropore distribution and effective size of the channels of natural sepiolite from Turkey were measured by the BET method. Before the BET measurement, the samples were treated under a water vapour atmosphere at various pressures to fill progressively the sepiolite micropores with water. The surface areas measured by means of N2 adsorption decreased with increased vapour pressures of water. The outer surface area was estimated by comparison of the surface area of the vacuum-dried sepiolite with that filled with adsorbed water. The total surface area was ∼290 m2/g, and the outer surface area was 170 m2/g, the difference being attributed to the structural micropores of the sepiolite. The ratio of the surface areas possessed by the channels and that of the outer surface suggest that the mean thickness of the sepiolite fibre was ∼12 nm. The effective size of the channels was estimated from the number of various-sized molecules sorbed by the sepiolite, the results showing that molecules larger than benzene could not migrate into the channels.

The thermal behaviour of sepiolite and palygorskite treated with butylamine was investigated by DTA, TG and EGA-MS under a flow of air and N2. It was shown that the amine was adsorbed by the clays, penetrating into the pores and replacing zeolitic and bound water. The presence of amine in the pores shifted the dehydroxylation peak to temperatures higher than for the untreated clays. Under N2 the thermal desorption of the amine from sepiolite was observed at 175, 275 and 525°C, and from palygorskite at 170 and 270°C. In addition to desorption, reactions of pyrolysis and condensation to charcoal were detected by the evolution of NH3, CH4 or H2, respectively. In air, oxidation of the organic matter led to the appearance of exothermic peaks, the temperatures of the most intense peaks being determined by the rate of C oxidation, but the shape of the DTA curves and the temperatures of exothermic shoulders were determined by the rate at which H from the organic molecules combined with O from the air.

It has been shown that clay minerals transform into (Mg, Al) or (Ni,Al) serpentine-like phases if treated hydrothermally in the presence of MgCO3 or NiCO3 (Shitov et al., 1974, Frank-Kamenetskii et al., 1978, 1983a,b; Kotel'nikova et al., 1976; Varela et al., 1983; Kotov et al., 1985; Ryumin et al., 1978) and that the polytypism of the initial kaolins is inherited by the products. It has also been shown that (Mg,Al) serpentine-like phases react with KC1 to form dioctahedral micas and Mg-rich serpentine phases, but the latter do not react to trioctahedral micas (Kotel'nikova et al., 1976). Consequently, it was desirable to look for mechanisms for obtaining trioctahedral micas from serpentine-like phases.